1. Field of the Invention
The present invention relates to a device for and method of controlling an air-fuel ratio of an internal combustion engine having a constitution to control an air-fuel ratio of a combustion mixture based on an oxygen quantity stored in a catalytic converter.
2. Related Art of the Invention
There has heretofore been known a device for controlling an air-fuel ratio having a constitution to estimate an oxygen quantity stored in a catalytic converter based on an air-fuel ratio detected by an oxygen sensor disposed on the upstream side of the catalytic converter and on an exhaust gas quantity, and to control an air-fuel ratio of a combustion mixture so that the oxygen quantity reaches a target value (refer to Japanese Unexamined Patent Publication Nos. 6-249028 and 10-184425).
When a misfire occurs in the engine, the atmospheric air directly flows into an exhaust pipe. Accordingly, the oxygen sensor detects a high oxygen concentration and from the detection result, it is judged that the oxygen quantity in the catalytic converter is increasingly changed.
When a misfire occurs, however, fuel injected into the engine flows without burned into the exhaust pipe and, hence, the unburned fuel is subjected to the oxidation reaction in the catalytic converter resulting in the much consumption of oxygen.
In practice, therefore, although the oxygen quantity is not largely changed to increase, an estimation value of the oxygen quantity is increasingly changed based on the detection result by the oxygen sensor. Accordingly, the air-fuel ratio is controlled to be rich so as to decrease the oxygen quantity thereby causing such a problem that the oxygen quantity is controlled to a value smaller than a target value.
When a misfire occurs, further, since a large quantity of fuel is subjected to the oxidation reaction in the catalytic converter, and the oxygen stored In the catalytic converter is abruptly consumed, an actual oxygen quantity is rather decreased.
Accordingly, if the air-fuel ratio is controlled to be rich based on the fact that the estimation value of oxygen quantity is increasingly changed on the basis of the result detected by the oxygen sensor, the reduction of oxygen quantity is further accelerated.
Besides, even if an erroneous control of air-fuel ratio can be avoided, a decreasing change in the actual oxygen quantity due to the oxidation reaction of fuel in the catalytic converter cannot be estimated from the exhaust gas quantity or the oxygen concentration. Accordingly, the actual oxygen quantity remains smaller than the target value causing an error in estimating the oxygen quantity.
It is therefore an object of the present invention to avoid that an unnecessary rich control operation is performed based on a detection result of oxygen concentration in the exhaust gas when a misfire has occurred.
It is another object of the present invention to quickly return an actual oxygen quantity back to a target value and to maintain estimation accuracy of oxygen quantity even in case the actual oxygen quantity is decreased due to the oxidation reaction of fuel in a catalytic converter when a misfire has occurred by.
In order to accomplish the above objects, according to the present invention, in a construction where an oxygen quantity stored in a catalytic converter is estimated based on an exhaust gas quantity and on an oxygen concentration in the exhaust gas, and a fuel injection quantity is feedback controlled so that the estimation value of oxygen quantity approaches a target quantity, when an occurrence of misfire is detected, a control of the fuel injection quantity based on the oxygen concentration detected at that time is prohibited.
Further, according to the present invention, the construction is such that, for a predetermined period of time from the end of misfire following the time when the occurrence of misfire is detected, the control of the fuel injection quantity based on the oxygen concentration detected at that time is also prohibited, and also the fuel injection quantity is forcibly corrected to decrease for the predetermined period of time from the end of misfire, and further, after the end of the decreasing correction, the estimated value of oxygen quantity is reset to a preset value.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.